
#ifndef __important_file_writer_h__
#define __important_file_writer_h__

#pragma once

#include <string>

#include "base/basic_types.h"
#include "base/file_path.h"
#include "base/memory/ref_counted.h"
#include "base/threading/non_thread_safe.h"
#include "base/time.h"
#include "base/timer.h"

namespace base
{
    class MessageLoopProxy;
    class Thread;
}

// Helper to ensure that a file won't be corrupted by the write (for example on
// application crash). Consider a naive way to save an important file F:
//
// 1. Open F for writing, truncating it.
// 2. Write new data to F.
//
// It's good when it works, but it gets very bad if step 2. doesn't complete.
// It can be caused by a crash, a computer hang, or a weird I/O error. And you
// end up with a broken file.
//
// To be safe, we don't start with writing directly to F. Instead, we write to
// to a temporary file. Only after that write is successful, we rename the
// temporary file to target filename.
//
// If you want to know more about this approach and ext3/ext4 fsync issues, see
// http://valhenson.livejournal.com/37921.html
class ImportantFileWriter : public base::NonThreadSafe
{
public:
    // Used by ScheduleSave to lazily provide the data to be saved. Allows us
    // to also batch data serializations.
    class DataSerializer
    {
    public:
        virtual ~DataSerializer() {}

        // Should put serialized string in |data| and return true on successful
        // serialization. Will be called on the same thread on which
        // ImportantFileWriter has been created.
        virtual bool SerializeData(std::string* data) = 0;
    };

    // Initialize the writer.
    // |path| is the name of file to write.
    // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
    // file I/O can be done.
    // All non-const methods, ctor and dtor must be called on the same thread.
    ImportantFileWriter(const FilePath& path,
        base::MessageLoopProxy* file_message_loop_proxy);

    // You have to ensure that there are no pending writes at the moment
    // of destruction.
    ~ImportantFileWriter();

    const FilePath& path() const { return path_; }

    // Returns true if there is a scheduled write pending which has not yet
    // been started.
    bool HasPendingWrite() const;

    // Save |data| to target filename. Does not block. If there is a pending write
    // scheduled by ScheduleWrite, it is cancelled.
    void WriteNow(const std::string& data);

    // Schedule a save to target filename. Data will be serialized and saved
    // to disk after the commit interval. If another ScheduleWrite is issued
    // before that, only one serialization and write to disk will happen, and
    // the most recent |serializer| will be used. This operation does not block.
    // |serializer| should remain valid through the lifetime of
    // ImportantFileWriter.
    void ScheduleWrite(DataSerializer* serializer);

    // Serialize data pending to be saved and execute write on backend thread.
    void DoScheduledWrite();

    base::TimeDelta commit_interval() const
    {
        return commit_interval_;
    }

    void set_commit_interval(const base::TimeDelta& interval)
    {
        commit_interval_ = interval;
    }

private:
    // Path being written to.
    const FilePath path_;

    // MessageLoopProxy for the thread on which file I/O can be done.
    scoped_refptr<base::MessageLoopProxy> file_message_loop_proxy_;

    // Timer used to schedule commit after ScheduleWrite.
    base::OneShotTimer<ImportantFileWriter> timer_;

    // Serializer which will provide the data to be saved.
    DataSerializer* serializer_;

    // Time delta after which scheduled data will be written to disk.
    base::TimeDelta commit_interval_;

    DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
};

#endif //__important_file_writer_h__